In an image forming apparatus of an electro-photographic type, an electrostatic latent image is formed on a photosensitive member by exposing the photosensitive member. Some exposing devices used for this exposure in recent years have an exposing head in which a plurality of light emitting elements such as LED is arrayed in the main scanning direction (the direction perpendicular to the direction in which a paper sheet is conveyed).
In this type of exposing head, a plurality of chips is arranged in the main scanning direction, each chip having a plurality of light emitting elements arrayed in the main scanning direction perpendicular to the sheet conveying direction. The light emitting elements in each chip are arrayed at generally accurate pitch with small manufacturing variations. On the other hand, there are manufacturing variations in the arrangement of the chips. Thus, the pitch between light emitting elements is not constant at joints of the chips. Hence, conventionally, the light amounts of light emitting elements are changed depending on the distance between the light emitting elements at a joint of chips, or a correction pattern is changed depending on the angle of a dither pattern having oblique lines relative to the main scanning direction, so as to suppress occurrence of a color streak (black streak) and a white streak in an image.